There is considerable evidence that the development of Hypertension is closely related to changes in reactivity of smooth muscle. Because calcium ions regulate cellular permeability and directly control resting tone and excitation-contraction coupling in cardiac, skeletal and smooth muscle, the integrity of calcium transport and of calcium homeostatic mechanisms are crucial to normal biological function. The objective of the proposed work is to utilize the internally dialyzed and voltage clamped axon of Myxicola infundibulum to develop a kinetic model of transmembrane calcium transport in excitable tissue. The giant axon offers a unique approach to the study of calcium transport since it allows control of intracellular ionic concentrations and of membrane potential which are not available to manipulation in multifibered preparations. The effects of changes of the transmembrane sodium gradient, of ATP, of potential difference, of temperature, of pH and of the substitution of divalent and monovalent cations upon calcium influx and efflux will be investigated. The kinetic model obtained from the giant axon will serve to provide more precise questions and approaches to homeostatic mechanisms governing calcium distribution in multicellular tissues, such as vascular smooth muscle.